Adjustable microwave attenuator



Un W68 at n ADJUSTABLE MICROWAVE ATTENUATOR John F. Zaleski, Thornwood, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application October 30, 1953, Serial No. 389,334 13 Claims. c1. s33-s1 This invention relates to adjustable attenuators employing longitudinal dissipative fins for use in microwave sipative material can take the form of a thin card or fin placed longitudinally in the guide. When the fin is so positioned as to be parallel to the electric field within the guide, and is so located as to be in the strongest part of the field, its dissipative efiect is greatest. This etfect is reduced in proportion as these two conditions are not met, with further complications in some cases due to the generation by the fin of higher modes of microwave transmission, to change induced by the fin of the plane of microwave polarization, and to the generation by the fin of reflections.

Adjustable attenuators usually move the fin into and out of the guide or move the fin between the maximum field region and a weak field region. In such attenuators the fin commonly is formed with a taper or an impedance matching step at each end so as not to'cause reflections toward the microwave generators The dissipative material of which the fin is made may consist of dielectric impregnated with carbon, or of va thin layer of carbon on a dielectric sheet. terials for .a precision attenuator for low powers is a very thin layer of a metal such as nickel-chromium alloy on a glass sheet. It is desirable that the fin be stable in both the reactive and the resistive components of its impedance. optimum specific resistance, for at both zero resistance and infinite resistance the dissipation is zero.

The limiting case in which the fin has substantially zero resistance, i. e., in which the fin is made of a low One of the best ma- Obviously there is for each attenuator an resistance metal such as copper or silver, has special h interest because althoughit has no function as a dissipative attenuator, it can function as an easily adjustable and highly eificient microwave phase changer.

The attenuator of this invention consists in'general of a thin dissipative fin having a length of one or several a wave-lengths in the guide (1 The fin has a width, for use in round guides, of a little less than one-half the diameter, or for use in rectangular guide its width is slightly less than the smaller cross sectional'dimension. For

use in coaxial guide the width'is sllightly less than the 2,830,275 Patented Apr. 8, 195 8 guide. The position of the hinge pin is so chosen that when it is rotated so that the fin is perpendicularto the peripheral guide wall, the fin is in the strongest part of the electric field set by up microwave energy transmitted through the guide, and is parallel to that component of the field. When the pin is rotated about from this position the fin is laid substantially parallel with the peripheral guide wall. Between these two positions the attenuator is continuously variable in its attenuationa .The maximum value of attenuation attainable with this design is unlimitedand is determined by the length of the fin. The minimum value of attenuation is very low. This low value of minimum attenuation constitutes a principal utility of this invention.

The principal purpose of this invention isto provide an improved microwave hollow guide low power adjust able attenuator.

Another purpose of this invention is to provide an adjustable attenuator having very low minimum attenu ation.

' A still further purpose of the invention is to provide an adjustable attenuator in which the moving parts are completely enclosed in metal and which contains no slots or other apertures through which microwave energy may be dissipated or radiated. 7

Further understanding of this invention maybe secured from the detailed description and the accompanying drawings, in which:

Figure 1 is a cross-section taken on line 11 of Fig. 2 of a rectangular hollow-microwave guide containing the attenuator of the invention.

Figure 2 is a side view of the attenuator cross-sectioned on the line 22 of Fig. 1. v

Referringnow to Figs. 1 and 2, a hollow rectangular guide 11 is made of brass silver plated on its interior surfaces. Its interior dimensions are suitable for the microwave frequency to be transmitted through it; for example, for a frequency of 9000 mc. p. s. the dimensions may be 0.4 inch in height and 0.9 inch in width. A slot inch wide and 1% inches long is cut in the center of the broad face 12 of the guide, and a rectangular pad 13 of brass is secured to the face surrounding'the slot. A brass block 14 is cut to fit the slot and is arranged to be screwed to the pad and guide by screws 16. A longitudinal hole .203 inch in diameter is drilled and reamed through the block 14 for the reception of a round brass rod 17. This rod is slightly smaller than the hole so as to have a running fit therein, and projects radially by somewhat less than its radius from the block. The rod is thus retained by the hole but can rotate therein. A transverse'slot 18 is cut at one end of block 14 and a handle 19 is inserted through it-and threaded. or pressed into the rod 17. The block is'split for almost its entire length by a slot 21 ending at the wall 22, Fig. 2. This slot 21 in combination with bolt 23 and self-locking nut 24 provides means for applying enough frictionbetwee n the reamed hole and rod 17! to hold the latter at any angle to which it is turned. To permit this adjustableaction the screws 16 pass through clearance holes in the block 14 and are threaded into the pad 13 and the top face 12 of the guide.

Alternatively by careful machinery of the parts the friction fit between the rod 17 and the reamed hole will provide sutficient friction to hold the fin 26 at any angle of adjustment. In such instances the slot 21 and clampl ing bolt 23 may be dispensed with.

,,The dissipative element consists of a fin or card 26 secured in an off-center slot 27 in the rod 17. The loca-.

.. canlbe positioned as indicated by the solid -cross;sec-. tion 26, Fig. '1, perpendicular to the broad sides of the guide, but not touching the guide wall 28, and also by rotating rod 17 in the position 29exactly parallel to the the tapered ends to constitute as little impedance dis continuity as may be required. Asa general rule, the length of one taper, /2 11, if not less than 4%,, will cause little reflection of the impinging. microwave energy.

A taper length of 2M, will reduce the discontinuity enough for almost any application. The shape of the taper is preferably elliptical or circular as shown, or it may be linear. In place of tapered ends, the finmay have ends cut to serve as impedance transformers. Such impedance steps are widely used, and take less space than tapers. I

A reactive effect is produced by the fin, which because of its conductance may-be considered to be a special form of tuning screw or probeextending into the guide parallel to the electric field. Such a screw or post acts essentially as a shunting reactance and if shorter than a is capacitive, the susceptance increasing as the length of the post increases from zero to A In the design of the attenuator described in connection with Figs. 1 and 2, the incident energy at any setting is divided into energy passed, energy dissipated as heat, and energy reflected. Energy reflection is undesirable, and at minimum setting it constitutes the greater part of the attenuation. Reflection may be partly due to the small residual discontinuity constituted-by thetapered end of the fin facing the power source, which can be minimized as much as desired by elongating the taper. The edge of the dissipative fin should be sharp to minimize reflection from that source. In order to eliminate reflection from the end of the hinge pin 17 facing the power source it should be terminated in some manner, two ways being depicted in Fig. 2. In one method a semicone of metal 31 is secured tofthe inner-face of wall 12 in prolongation of the hinge pin 17. The pointed guard 31 is separated from metallic rubbing contact with the end of hinge pin 17, which mightcause spar-king, by a spacer 32 of a dielectric material such as tetrafluoroethylene. In another method involving phase shift neutralization a short metallic post 33 ispositioned in line with the end of. binge pin 17 and ate distance of from (it, N being any integer;

The attenuator as described is completely enclosed in metal and there are no slots or otheriapentures extending from the interior through to the exterior of the device so that no microwave energy is radiated therefrom. When a good friction fit is provided between the rod 17 and the block 14 the slot 21 may be dispensed with' a's described above and the rod or hinge pin 17 may then be composed of dielectric material. In such instances the tapered guard 31 should also be made of dielectric material. The phase shift neutralizing post 33- can be made either of conductive or ofdielectric material.

Inoperation, when the dissipative fin is in the position it to be dissipated as heat. The amount of effect depends upon the length of the fin and upon its width. A maximum is set to the permissible width by the necessity of providing a clearance gap 34 between fin 26 and the broad floor 28 of the guide sufficient to prevent sparking and also for mechanical operating clearance.

In the position of the dashed lines 29 the fin short circuits the electric field only in the proportion of its thickness to the dimension b of the guide. If the dissipative member is a thin film of material its thickness is completely negligible compared to the dimension [7 and contributes nothing to the minimum attenuation attained with the fin in this position.

In intermediate positions of the fin the degree of attenuation is dependent upon the length of the projection of the fin upon a plane parallel to the narrow guide walls, and is closely proportional to the square of the sine of the angle between the fin and theposition 29.

When the attenuator is used in coaxial or round hollow guide the fin should be curved, so that in the minimum attenuation position the fin follows the curvature of the periphery of the guide and is perpendicular at all points to the electric field direction.

What is claimed is:

l. A device of the class described comprising, a waveguide transmission section, an elongated adjustable hinge member positioned in the plane of a wall of said waveguide section parallel to the longitudinal axis thereof, a fin member afiixed to said hinge member and extending into the interior of said waveguide section, and means for rotating said hinge member about its longitudinal axis whereby said fin member is adjusted between a position parallel to the electric field within the waveguide section and a position perpendicular thereto.

2. A device of the class described comprising a hollow waveguide transmission section, a pintle member positioned in the plane of a wall of said waveguide section parallel to the longitudinal axis thereof and having a portion of its surface extending into the interior of said waveguide section, an elongated sheet member afiixed to said pintle member and lying in a plane parallel to the axis thereof, and means for rotating said pintle memher about its longitudinal axis whereby said sheet member is adjusted between a position parallel to the electric held within the waveguide section and a position perpendicular thereto. I

3. A device as set forth in. claim 2 in which said sheet member is composed of conductive material and is pro- Vided with a resistive coating on the face opposite to that adjacent the waveguide wall when the sheet membet is adjusted to the position perpendicular to the electric field.

4. A device of the class described comprising, a hollow waveguide transmission section, a pintle member positioned in the plane of a wall of said waveguide section parallel to the longitudinal axis thereof with a portion thereof extending into the interior of said waveguide section, an elongated sheet member having an edge thereof affixed to the portion of. said pintle member extending into said waveguide section and lying in a plane parallel to the axis thereof, at least one end of said sheet member being provided with means for preventing impedance discontinuity and consequent: reflection of impinging microwave energy, and means for rotating said pintle member about its longitudinal axis whereby said shect member; is rotated therewith between a position parallel with the electric field within the waveguide section and a position perpendicular thereto.

5. Adevice'as setifor'th in claim 4 in which said sheet member is composed of conductive material provided with a resistive coating in'the face thereof opposite to that adjacent the waveguide wall when the sheet member is positioned perpendicularly to the electric field.

.6. A device of theclass described comprising a, hollow waveguide transmission section, a pintle member positioned in the plane of a wall of said wave guide member parallel to the longitudinal axis thereof with a portion less than the radius thereof extending into the interior of said waveguide section, a sheet member having a straight edge intersected by an arcuate edge, said sheet member having its straight edge portion fixed to the portion of said pintle member extending into said waveguide section, and means for rotating said pintle member about its axis whereby said sheet member is rocked from a position parallel to the electric field within said waveguide to a position perpendicular thereto.

7. A device as set forth in claim 6 in which said sheet I member is composed of conductive material provided with a resistive coating on the face opposite to that which lies adjacent the waveguide wall when the sheet is adjusted to one of its limit positions.

8. A device of the class described comprising, a hollow rectangular waveguide transmission section, a pintle member positioned in the plane of a broad face thereof with its axis parallel to the longitudinal axis of said waveguide section, an elongated sheet member having an edge affixed to said pintle member and positioned interiorly of said waveguide section in a plane parallel to the axis thereof, at least one end of said sheet member being provided with means for preventing impedance discontinuity and consequent reflection of impinging micro wave energy and means for rotating said pintle member about its longitudinal axis whereby said sheet member is rotated therewith from a position extending across the narrow dimension of said waveguide section to a position parallel with a broad face thereof.

9. A device as set forth in claim 8 in which said sheet member is composed of conductive material provided with a resistive coating on the face opposite to that adjacent the waveguide Wall when the sheet is adjusted to its position parallel to the broad face of said Waveguide section.

10. A device of the class described comprising, a hol low rectangular waveguide transmission section, a pintle member positioned in the plane of a broad face'thereof with its axis parallel to the longitudinal axis of said waveguide section, a sheet member having a straight edge and an arcuate edge intersecting. said straight edge at the ends thereof, said sheet member having its straight edge fastened to said pintle member along the length thereof to extend into the interior of said waveguide section and having a maximum width between said straight edge and said arcuate edge slightly less than the narrowwidth of said waveguide section, and means for rotating said pintle member whereby said sheet member is rotated therewith between a position normal to the broad face of said waveguide section to a position parallel therewith.

11. A device as set forth in claim 10 in which said sheet member is composed of conductive material provided with a resistive coating on the face opposite to that which lies adjacent to the waveguide wall when the sheet is adjusted to the position parallel to the broad face thereof.

12. A device of the class described comprising, a hollow rectangular waveguide section provided with an elongated slot in the center of one broad face and extending in a direction parallel to the longitudinal axis thereof, a block member having a face provided with a projecting portion fitted into and filling said slot, said projecting portion being provided with a round concave groove extending along the length thereof, the depth of which exceeds its radius, a rod fitted into said groove, a sheet member having a straight edge aflixed to said rod along the length thereof and having a convex edge extending into the interior of said waveguide section, the maximum distance between said straight edge and said convex edge being slightly less than the internal narrow dimension of said waveguide section, and means for rotating said rod in said groove whereby said sheet member is rotated therewith between a position normal to the broad face of said waveguide section to a position parallel therewith.

13. A device as set forth in claim 12 in which said I sheet member is composed of conductive material having the face opposite to that which lies adjacent the broad waveguide wall when the sheet .is adjusted to the position parallel thereto, coated with a resistive material.

References Cited in the file of this patent UNITED STATES PATENTS 2,433,368 Johnson et al. Dec. 30, 1947 2,570,813 Ioerndt Oct. 9, 1951 2,603,710 Bowen July 15, 1952 2,646,551 Roberts July 21, 1953 FOREIGN PATENTS 608,494 Great Britain Sept. 16, 1948 641,227 Great Britain Aug. 9, 1950 692,308 Great Britain June 3, 1953 

